1. Field of the Invention
This invention relates to the formation of a cathodic protective coating on magnesium or magnesium alloys and to the hydride or hydrogen-rich coating so formed. More specifically, such coating is produced by an electrochemical treatment in an alkaline bath containing hydroxide and supporting electrolytes with use of a source of cathodic current.
2. Brief Description of the Prior Art
Magnesium alloys have been increasingly utilized in structural applications. By minimizing metallic impurities and adding aluminum or rare-earth elements, the corrosion rates of magnesium alloys become comparable to those of carbon steels or A380 aluminum alloys in salt spray environment. Painting is a popular method to improve the corrosion resistance and to add decorative appearances. Chemical or electrochemical pretreatment is usually applied before painting to strengthen the adhesion between the paint film and Mg surface. These treatments also provide limited corrosion protection. Among them, chromium (VI) compound based chemical conversion coatings are known to offer a good paint base. However, because of its toxic nature, the handling of the solution and its disposal are of concern. As such, several non-chromium (VI) based coatings such as zirconium- or permanganate-based coatings have been developed (e.g. U.S. Pat. No. 5,380,374 of Jan. 10, 1995 entitled "CONVERSION COATINGS FOR METAL SURFACES"). These surface coatings, including chromium based coatings, usually require regular control of chemical composition, as chemicals are consumed during the operation.
Another electrochemical surface treatment of magnesium or its alloys is called "anodizing" or "anodization" and involves formation by anodic deposition of an oxide/hydroxide or similar protective film or coating on the magnesium article. Examples of such treatments are disclosed, for example, in U.S. Pat. Nos. 2,314,341 and 2,426,254. There are also two-step processes where the magnesium article is first pre-treated in a chemical or electrochemical solution, before being subjected to the anodic deposition of the protective coating. Examples of such two-step processes may be found in U.S. Pat. Nos. 5,240,589 and 5,264,113. These processes employ an anodic technique, i.e. the Mg substrate is polarized to a more positive voltage.